In wireless communication, the actual channel through which a signal is transmitted differs in dependence on the physical location of the transmitter and receiver and objects present in the environment. For instance, when a receiver is in a shadow, the receive signal strength can be very low. Wireless communication systems are usually designed to include a capability of measuring and estimating the channel and adapting the transmission settings accordingly so that a reliable transmission can be achieved.
For a receiver to decode a received signal into bits correctly, the receiver needs to know the transmission settings, among others, the modulation format, coding scheme and rate, and the amount of data transmitted. These settings are set by the transmitter taking into account the actual transmission channels to each receiver, which may only be available to the transmitter at the end of a channel reservation mechanism. On the other hand, the settings chosen by the transmitter must be known by the receivers prior to data transmission. This means that these settings should be communicated in a transmission with known/fixed transmission settings preceding the actual data transmission.
A potential for further bit-rate increases is seen in a use of multiple-input multiple-output (MIMO) antenna systems. To this end, a new medium access control (MAC) protocol mechanism has been proposed, which supports multi-user (MU) MIMO transmissions in WLANs according to IEEE 802.11 based standards. The proposed new protocol extends the single-user (SU) MIMO protocol in such a way that different stations can be destination stations for packets inside a MIMO frame (which is a set of packets transmitted simultaneously on different spatial streams). In ubiquitous networking, a station might thus be communicating with multiple other users at a time.
In wireless systems, such as for example 802.11a/g/n systems, data transmission settings can be transmitted in a dedicated field, e.g. the SIGNAL field in the 802.11a/g/n system, of the preamble of the transmission, after the transmission of training sequence for acquisition, synchronization, and channel estimation. However, the known systems are single-user systems.